So Humon asked which monsters people find the most scariest? Options were:

zombies
aliens
demons
ghosts
werewolves
something else

The answer for me is, first place: the aliens from Alien and Aliens movies. And I love them at the same time. And I'm kind of horrified that a human thought them up.

Second place: humans monsters, the human capacity for evil. People like Hitler, Nazis,  scientists without reason/conscience/empathy, religious zealots, sociopaths etc. etc. are a lot more scary than any supernatural beings. Because they can hide behind the face of your mother, neighbor, child, co-worker, lover etc. and appear totally normal, nice people.